State of the art digital cameras used in mobile phones may typically be used at light levels down to 5 to 10 lux. The image quality under such low light conditions may be poor, however.
Mobile phones of the state of the art typically include a white light emitting diode (LED) that may be used as an assisting light while capturing images. The LED makes it possible to use the camera even in complete darkness.
Some phones may even have a Xenon flash (discharge tube). The Xenon flash solution is considered a suitable choice from a technical/image quality point of view. Currently, a Xenon flash can only be used for still images, however. While the white LED solution can be used for still images and/or video recording/video conferencing, there are several disadvantages with the white LED solution. For example, the range is limited to about 0.5 m (in continuous use) due to power consumption and/or heat dissipation problems, which may be a severe limitation for video recording. While 0.5 m may be sufficient for video conferencing where the main purpose is to show the person making the call, another problem may arise. A strong white LED aimed directly towards a person's eyes may be very annoying. The user may also like to avoid attention when using the phone in a public environment. The video conferencing camera is usually aimed in the opposite direction to the phones main camera and cannot take advantage of the main camera's LED. The cost for an extra LED may rival the cost for the complete video camera itself.
Another possibility for recording in low light conditions is the night vision technology known from Sony video cameras as well as from surveillance equipment. This technique takes advantage of the fact that complementary metal-oxide-semiconductor (CMOS) and charge-coupled device (CCD) image sensors are very sensitive to infrared radiation (IR). A digital camera intended for normal use has an infrared cut off filter in order to reduce the cameras sensitivity to visible radiation. Without this filter the color rendition becomes poor.
A camera with a vision switch, such as a Sony video camera, has a removable IR-cut off filter. The filter is moved in or out of the optical path by some mechanical means. The camera is usually equipped with an IR-LED which lights up the scene with radiation invisible to the human eye. An IR-LED in combination with a night vision camera is much more efficient than a white LED used with a standard color camera. An obvious drawback with night vision is that only black and white pictures can be produced.
The implementation of a night vision camera in a mobile phone is more difficult due to the miniaturization of the camera and the high standards of robustness that may be required for mobile phones. A moveable filter is technically feasible, but adds cost, size and risk of mechanical failure. A further problem with the moveable filter solution is the fact that color cameras have color mosaic filters attached to each pixel of the sensor. The red, green, and blue filters are necessary for taking color pictures, but may reduce the efficiency of the night vision mode. This color filter array is an integrated part of the sensor and it cannot be removed.